Here is another view of having used a small food vacuum machine to gather the bag up snug.
The guitar is then placed into it's inner carton and firm newspaper nesting is applied under the body, neck, headstock and sides ... (After this step, wadded newspaper is placed over the guitar as well):
Pulling the inner cartons sides up snug compresses the newspaper-packing into a nice nest that prevents the guitar from moving in the inner carton while the inner carton acts as a protective baffle to keep the guitar and inner carton from moving in it's outer carton shown on the next page:
Inner carton seen from the top / lid side; Ready for packing into a custom cut outer carton: